The invention relates to an antenna feed system for double polarization in two high-frequency bands of differing frequency position, consisting of a polarization dividing filter having an antenna-end terminal which is common to the two frequency bands and having two directional antennae which are each assigned to one polarization direction in each case for a frequency dividing filter whose terminal which is common to the two frequency bands is in each case connected to one of the directional antennae of the polarization dividing filter, further consisting of a first 3-dB-directional coupler for the lower frequency band, whose double access is connected to a further terminal, assigned to the lower frequency band, of the two frequency filters, and further consisting of a 3-dB-directional coupler for the upper frequency band which is connected to a further terminal, assigned to the upper frequency band, of the frequency filters.
An antenna feed system of this type is known, for example, from the publication "Proceedings of Intelsat 5 Earth-Station Technology Seminar" in Munich of the 13th to 18th June, 1976, in particular FIG. K-15. FIG. 1 is a block circuit diagram of an antenna feed system of this type for circular double polarization in two frequency ranges. The fundamental aim of such an arrangement consists in converting two transmitting bands of like frequency position, for example, from 5.925 GHz to 6.425 GHz and with powers of up to approximately 10 KW into a transmitting band of right-circular wave form and a further transmitting band of left-circular wave form, and thus to provide that the latter, again decoupled from one another, are combined in a common main waveguide which also conducts two like-frequency receiving bands with a frequency position which is displaced relative to the transmitting bands, for example from 3.7 GHz to 4.2 GHz in right-circular and left-circular polarized wave form, thus decoupled from one another. These receiving bands are also separated in respect of their right-circular and left-circular polarization and, having been converted into the H.sub.01 wave form, must be fed to two receiving waveguides.
These functions are fulfilled by the circuit illustrated in FIG. 1 in the following manner. For example, a 6 GHz band is split by means of a 3-dB-directional coupler into two half waves with +90.degree. and -90.degree. phase difference. The sign of the 90.degree.-phase is dependent only upon which of the two arms of the 3-dB-coupler at which feed-in takes place. These half waves are fed via two identical frequency filters to a polarization filter in such a manner that they are at right angles to one another at the latter's output. If the condition is fulfilled that the two half waves travel their path to the polarization filter output without mutual phase distortion, at said output they still possess a mutual time phase of .+-.90.degree. and thus represent a purely circularly polarized wave.
The aim of the circuit illustrated in FIG. 1 is, employing one and the same antenna, to radiate two like-frequency transmitting bands right-hand circularly and left-hand circularly, thus decoupled from one another, and to feed two right-hand circular and left-hand circular waves received by this antenna in a different frequency band to separate receiving amplifiers in accordance with their polarization direction.
The difficulties which occur in the realization of the concept illustrated in FIG. 1 mainly consist in designing the two transit paths, each provided for one frequency band, of the overall arrangement to be symmetrical in construction or at least symmetrical in phase. Furthermore, it is to be endeavored to provide the best possible transmission properties in respect to attenuation, reflection and decoupling for the four transit paths of the circuit illustrated in FIG. 1.